What can I put in the bottom of a tall planter to make it moveable?

empty plastic milkjugs and or Styrofoam peanuts, bubble wrap wadded up.

I use empty water bottles or soda bottles with the caps on, whatever you have on hand. You could also use empty ketchup bottles or such as long as you wash them out good and keep the caps on and closed tightly. Without the caps, they will collect water and get heavy defeating the purpose. Good luck and happy gardening!

I used pine cones to fill my large planters. Works great.

You can buy planter holders with wheels; I use those for my large houseplants.

I use wine corks.

Packing peanuts

Thanks all! I have acquired some styrofoam and chopped it up for the bottom of the plants. Was afraid there was something in that to hurt the plants but with that many suggestions for it, it must be great. Hugs to all!